how to make an object like a flat?
The reason this happens is because you have 'subsurface' modifier enabled.
What subsurface modifier does is basically adds extra geometry to your mesh.

In your case, the type is set to Catmull-Clark which in short will basically smooth out your mesh.

So if you start with a basic cube 



and apply the modifier on it, it will smooth it out and it will start to resemble a circle 


Now an easy way to make the object look like what you want it to be is add extra vertices where you want the detail. So if I were to add some extra vertices at the corners of the cube it will start to resemble the cube again


Now if you understood this you would have guessed you need to add extra geometry in your mesh at the top of your coffee(or water? depends on how far you are in that tutorial)

An easy way to add geometry to a closed loop is the inset tool(hotkey I), this tool basically makes another smaller loop inside the loop and connects the vertices.